Ellipsometry is a technique which measures the change in polarization of light when reflected off a sample surface. It derives its sensitivity from the determination of the relative phase shift Δ between normal (p) and in-plane (s) components of polarization vector in the reflected light. The phase shift can be measured with precision of 0.1 degree, which translates to around 1 nm of organic material on silicon substrate in air.
Precision ellipsometry (PREL) is based on modulation of polarization, which further increases sensitivity to 0.01 mrad, which translates to 0.01 nm. This high sensitivity allows real time measurement of atomic and molecular attachment and detachment.
FIG. 1 shows a prior art system that is used for precision ellipsometry. Not shown are a light source, which produces linearly polarized light that is made incident 104 on a substrate 106 providing the sample surface, and a polarization analyser and detector 108 placed on the path of the reflected beam 110.
A cuvette 120 can be used to introduce a layer, where any one or more of molecular binding, adsorption and desorption occurs, onto the substrate 106 surface. The cuvette 120 is of closed flow type and has an inlet 122 and an outlet 124 only for liquid. The substrate 106 must be placed within the cuvette 120 in advance before closing and optical alignment. While the closed flow cuvette 120 allows quick exchange of solvent and solution, it requires a long time to change the substrate 106, because this involves draining of liquid before opening, draining of air after closing and alignment of the optical analysis system (not shown) for each substrate 106.
FIG. 2 shows a prior art immersion cuvette 220. The immersion cuvette 220 of FIG. 2 has an inlet 222 and an outlet 224 for liquid and an opening 226 through which a substrate 206 can be immersed. The advantage of this cuvette 220 is the possibility of fast changing the substrate 206 together with keeping all other parts of the system stable. Thus when a next substrate (not shown) is immersed, the system is ready for the next measurement. However liquid flow pattern is complicated because the inlet 222 and the outlet 224 are introduced from the top of the cuvette 220. Hence a magnetic stirrer 227 is required to achieve uniformity of the solution, and hence it is suitable to measure only slow kinetics, e.g. in the range of minutes.
Another optical technique for measurement of molecular binding is surface plasmon resonance (SPR), which is shown in FIG. 3. SPR relies on a measurement of refractive index change at a metal surface, when molecules attach to the surface. It works as follows: light 304 enters through a transparent substrate 306 at incidence angle θSPR. The light 304 induces plasmon resonance in a gold (Au) layer 315; deposition of the layer 313 changes the value of the resonance angle. If incidence angle θSPR is fixed, it changes reflected intensity. Because surface plasmon wave is evanescent in the direction normal to the surface and propagates along the surface, it is very sensitive to the molecules near the surface and less sensitive to molecules far from the surface, or changes in the medium. However, SPR only works with noble metals (mostly gold), which limits the range of applications of this measurement technique.
There is thus a need to address the shortfalls of the measurement techniques described above.